Non-reflective/reflective phase transition optical modulator

ABSTRACT

An optical modulator particularly suited for use as an output coupler from the resonating cavity of a laser. The modulator is fabricated with a semiconducting oxide compound selected from the group consisting of VO, VO 2 , V 2  O 3 , V 3  O 5 , V 4  O 6 , V 5  O 9 , V 6  O 11 , V 7  O 13 , V 8  O 15 , Ti 2  O 3 , Ti 4  O 7 , NbO 2 , FeSi 2 , VO 2  NbO 2 , V 1-x  Mo x  O 2 , VO 2  -TiO 2 , V 2  O 3  -Ti 2  O 3 , V 1-x  Ge x  O 2 , V 1-x  Nb x  O 2 , V 1-x  Cr x  O 2 , (Cr x  V 1-x ) 2  O 3  and V 1-x  Ti x  O 2 . When an electric field is applied to an output coupler fabricated with a thin film or single crystal of material from the foregoing group, the material undergoes a transition between metallic and semiconducting states. In the metallic state the output coupler reflects incident luminous energy. In the semiconducting state, on the other hand, the material transmits such luminous energy. Thereby, a laser system output coupler fabricated with a material from the above-defined group can, by external stimulation, be caused to reflect back or transmit out the laser energy stored within the resonating cavity. Furthermore, since transition between states can be performed with speeds ranging from microseconds to nanoseconds, the output coupler is compatible with the needs of Q-switched or cavity dumped laser configurations.

RIGHTS OF THE GOVERNMENT

The invention described herein may be manufactured and used by or forthe Government of the United States for all governmental purposeswithout the payment of any royalty.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of copending applicationassigned Ser. No. 06/139,070 and filed Apr. 10, 1980, now abandoned,which was a continuation-in-part of an application assigned Ser. No.965,464 and filed on Nov. 30, 1978, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention disclosed herein relates to modulators of luminous energytypified by that generated in lasers, and particularly those modulatorsfabricated with oxide compounds which undergo transitions betweenmetallic and non-metallic states in response to external stimuli.

2. Description of the Prior Art

Modulators of luminous energy radiating in the infrared, visible andultraviolet spectra are, as a general premise, known by those skilled inthe related arts. Likewise, lasers and means for their control aregenerally known. What is lacking is a laser system luminous energymodulator which can be inserted into the output path of a laser andoperate as a reflective mirror or output coupler, translating betweenstates at the behest of an external control with sufficient speed togenerate pulsed laser outputs.

The related art known to the inventor appears in U.S. Pat. No. 3,455,627granted to inventor Eugene C. Letter and U.S. Pat. No. 3,656,836 grantedto joint inventors Baudoin de Cremoux and Pierre Leclerc. The teachingsof the patent to Letter disclose an optical element which alternatesbetween reflecting and absorbing states in response to the thermaleffects of high energy density luminous radiation. The transition is aself-initiated thermal phenomenon, with the device reverting to itsprior state upon the dissipation of the thermal energy added by theincident radiation.

The other patent, de Cremoux et al, also addresses the modulation ofluminous energy. The device disclosed in this patent is defined toundergo transition between a first transmission state and a secondabsorption state upon the formation of an electric field within thesemiconductor material constituting the device. Thereby, the modulationcharacteristics of this device are readily amenable to external control.

Neither of the foregoing disclosures describe a modulating deviceparticularly suited to the job of extracting luminous energy from theresonating cavity of a laser. Notwithstanding the multiplicity of knowntechniques for accomplishing such an objective, none approach thesimplicity or efficiency of a device which can be induced to undergotransitions between highly reflective and transmissive states.

SUMMARY OF THE INVENTION

The subject matter of this invention is a particular group ofsemiconducting materials and their implementation to controllablyextract luminous energy from the resonating cavity of a laser. Unlikethe many complex and often inefficient techniques utilized by thosepracticing in the art, a device appropriately fabricated with one ormore of the semiconductor materials from a group consisting of VO, VO₂,V₂ O₃, V₃ O₅, V₄ O₇, V₅ O₉, V₆ O₁₁, V₇ O₁₃, V₈ O₁₅, Ti₂ O₃, Ti₄ O₇,NbO₂, FeSi₂, VO₂ NbO₂, V_(1-x) Mo_(x) O₂, VO₂ -TiO₂, V₂ O₃ -Ti₂ O₃,V_(1-x) Ge_(x) O₂, V_(1-x) Nb_(x) O₂, V_(1-x) Cr_(x) O₂ ; (Cr_(x)V_(1-x))₂ O₃ and V_(1-x) Ti_(x) O₂ can perform analogous operations. Theoxide compounds forming the group are generally characterized by theirnarrow 3d bands and, typically, energy gaps within the range extendingfrom 0.1 eV to 0.4 eV. A further common trait is their periodic tablelocation, wherein the compounds tend to be oxides of elements in eithergroup VB or IVB.

The compounds constituting this group undergo a phase transition inresponse to externally introduced stimuli of thermal energy, hydrostaticpressure or electric field. The phase change exhibited is betweenmetallic and non-metallic states, which, as a general rule, arecoextensive with their being highly reflective or nonreflective(transmissive) to incident luminous energy.

When a compound from this group is used in the fabrication of an outputcoupler for a laser resonating cavity, it functions as the cavity mirrorwhile in the reflective state and as a substantially transparent outputwindow when stimulated into its transmissive state. The simplicity ofthis single device is strikingly apparent when it is compared to theconventional methods of extracting laser energy from the gain medium ofa laser resonating cavity.

The semiconducting oxide compounds from the above defined group may befabricated as individual crystals or thin films, the latter beingdeposited onto a transparent substrate or sandwiched between layersthereof.

Another characteristic exhibited by this group of compounds, furtheraccentuating their suitability for laser applications, resides in theabrupt rate of the transition between states. When appropriatelycontrolled by means of the prescribed stimuli, pulse widths ranging frommicroseconds to nanoseconds, with repetition rates in the range ofmegahertz, are fully contemplated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a and 1b schematically show the respective layouts of aconventional Q-switched laser and a conventional cavity dumped laser.

FIG. 2a schematically depicts a single crystal structure fabricated fromone of the oxide compounds in the group.

FIG. 2b is a schematic of another embodiment, wherein a thin film of thecompound is sandwiched between transparent materials.

FIGS. 3a through 3d consist of four schematics embodying differing formsof stimuli; in which 3a shows radial hydrostatic pressure to anindividual crystal, 3b shows the use of hydrostatic pressure with a thinfilm sandwich, 3c depicts the application of thermal energy, and 3dshows a thin film structure stimulated by an electric field.

FIG. 4 contains a schematic depiction of a thin film sandwich deviceused as a laser output coupler at the control of an external voltagesource.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIGS. 1a and 1b schematically represent two of the many conventionallaser configurations, these two being known in the parlance of the artas a Q-switched laser and a cavity dumped laser, respectively. In theQ-switched laser of FIG. 1a rotating mirror 1 is synchronized with thetransmission of luminous energy 3 from flash lamp type pump 2 into gainmedium 4 of the laser's resonating cavity. When mirror 1 rotates intoalignment, lasing commences. Output coupling mirror 6 normally transmitsabout 30% of the incident luminous energy, reflecting the remainder backinto gain medium 4. Thereby, laser output 7 consists of pulses having arepetition rate corresponding to the rotation rate of mirror 1.

The cavity dumped configuration in FIG. 1b extracts a laser pulse bydeflecting the luminous energy beam. As with the Q-switchedconfiguration pump 2 supplies luminous energy 3 to gain medium 4 of thelaser resonating cavity. At one end of gain medium 4 is stationarymirror 8, while the luminous path from the other end of gain medium 4communicates with mirror 9, acousto-optic modulator 11, mirror 12, andat times prism 13. Since the configuration in FIG. 1b is well known, itneed only be noted that stimulation of acousto-optic modulator 11 altersthe path of incident luminous energy onto the path defined by referenceline 16. Without stimulation, luminous energy passing throughacousto-optic modulator 11 follows the path defined by dashed line 14,effectively being retroreflective as to gain medium 4.

The central feature of the invention is a utilization of oxide compoundsfrom a defined group to directly control the luminous energy leaving theresonating cavity of a laser. When stimulated, compounds from this grouprespond by undergoing a transition between metallic and non-metallicstates, which correspond to reflection or transmission of the incidentluminous energy.

The compounds to which this invention pertains are generallysemiconducting oxides of elements found in Group VB or IVB of theperiodic table, and are further characterized by their narrow 3d bandsand energy gaps ranging from 0.1 to 0.4 ev. In particular, the groupconsists of VO, VO₂, V₂ O₃, V₄ O₇, V₅ O₉, V₆ O₁₁, V₇ O₁₃, V₈ O₁₅, Ti₂O₃, Ti₄ O₇, V₃ O₅, NbO₂, FeSi₂, VO₂ NbO₂, V_(1-x) Mo_(x) O₂, VO₂ -TiO₂,V₂ O₃ -Ti₂ O₃, V_(1-x) Ge_(x) O₂, V_(1-x) Nb_(x) O₂, V_(1-x) Cr_(x) O₂,(Cr_(x) V_(1-x))₂ O₃ and V_(1-x) Ti_(x) O₂.

Structurally, the materials may be fabricated in the form of a singlecrystal, as shown by reference 17 in FIG. 2a, or may take the form of athin film, such as thin film 18 sandwiched between layers 19 and 21 oftransparent material. The latter embodiment is schematically depicted inFIG. 2b.

All materials in the defined group exhibit a transition phenomenon inresponse to externally introduced stimuli of hydrostatic pressure,thermal energy or electric field. FIG. 3a depicts the application ofradial pressure 22 to single crystal 17. The stimulation in FIG. 3b isanalogous, where hydrostatic pressure 24 is applied to exterior layers19 and 21 to compress thin film 18 sandwiched therebetween. The use ofthermal energy 26, depicted schematically in FIG. 3c, can be used witheither structural form.

Although it is recognized that the transition will occur under animposed thermal or pressure stimulus, the utility of the output couplerof the present invention resides in its ability to transition atcontrollably repetetive rates in the megahertz range under an appliedelectrical stimulus. Therefore, FIG. 3d, shows the preferred embodimenthereof in the form of a thin film, 18, of a compound from the groupsandwiched between transparent exterior layers 19 and 21. This schematicalso shows the presence of two electrically conductive intermediatelayers 25 and 30, and an external voltage supply 27 attached thereto.Conductive layers 25 and 30 would preferably be thin electricallyconducting films fabricated from materials which are transparent at thewavelength of the luminous energy transmitted through the device. Forexample if operation is desired in the infrared region of the spectrum,conductors made of In₂ O₃ or Sb would be appropriate.

The application of voltage 27 in the manner described creates anelectric field across thin film 18 of the transition material. As anexample of the relative magnitudes of voltage needed to inducetransition, consider the compound VO₂. For a thin film of thisparticular material used in a relatively small output coupler voltagemagnitudes ranging from 10 to 50 volts and electric current flows from 1to 50 milliamperes are typical of that which is necessary to trigger aphase transition. Furthermore, since the transition between states isvery abrupt, voltage source 27 is not restricted to a fixed value butmay, when the application demands, include repetitive short durationpulses, either synchronized or free running.

Stimulation by the imposition of an electric field is not limited tostructures in which the oxide compounds take the form of a thin film. Ifpatterns of electrodes are plated or otherwise deposited onto two facesof a single crystal structure, the composite is amenable to stimulationby the electric field formed when a voltage is applied between theelectrodes.

As noted previously, the central feature of the invention resides in theutilization of the above-described materials and devices to modulate,and thereby control, the luminous energy output from a laser resonatingcavity. A schematic representation of one such embodiment appears inFIG. 4, where output coupler 28 is shown to be of the thin film sandwichconfiguration. Voltage control 29 is analogous to voltage source 27 inFIG. 3d, altered as necessary to synchronize with pump 2.

Lasing of the resonating cavity in FIG. 4 commences when pump 2 hasintroduced sufficient energy 3 into gain medium 4 and output coupler 28is stimulated into a reflective state. Luminous energy output 31 appearsafter voltage control 28 alters the stimulus to output coupler 28sufficiently to cause a transition from the reflective state into thetransmissive state. The configuration depicted in FIG. 4, and describedwith reference thereto, is operated in the mode commonly known as cavitydumped. Equally feasible is a Q-switched configuration of the generalform depicted in FIG. 1a, in which the reflective function of rotatingmirror 1 is performed by a phase transition material coupler element inchanging from its transmissive state to its metallic state. In eithercase, the luminous energy output from the laser cavity is in the form ofrepetitive pulses, the rate of which is determined by the transitiontime between the reflective and non-reflective couplers rates.

Those skilled in the art will recognize the degree of simplificationattained when a single output coupler, 28, replaces the plethora ofoptical and mechanical devices previously needed to regulate theluminous energy output from a comparable laser resonating cavity. Asingle device now accomplishes that which took rotating apertures, Kerrcells, Pocket effect crystals, dyes or destructable films inconventional lasers. Furthermore, the output coupler undergoestransition at the behest of an external control, and as such is operablerepeatedly without degradation, thus being in distinct contrast touncontrollable output coupling materials, such as germanium, whichdeteriorate rapidly with successive output pulses.

I claim:
 1. An output coupler for the resonating cavity of arepetitively pulsed laser, comprising:a. an optical device forcontrollably reflecting and transmitting luminous energy repetitively inresponse to an externally applied electric field, said device comprisinga material selected from the group consisting of VO, VO₂, V₂ O₃, V₃ O₅,V₄ O₇, V₅ O₉, V₆ O₁₁, V₇ O₁₃, V₈ O₁₅, Ti₂ O₃, Ti₄ O₇, NbO₂, FeSi₂, VO₂NbO₂, V_(1-x) Mo_(x) O₂, VO₂ -TiO₂, V₂ O₃ -Ti₂ O₃, V_(1-x) Ge_(x) O₂,V_(1-x) Nb_(x) O₂, V_(1-x) Cr_(x) O₂, (Cr_(x) V_(1-x))₂ O₃ and V_(1-x)Ti_(x) O₂ ; and b. means for selectively applying an electric fieldacross said device whereby said optical device material is selectivelystimulated to transition between the metallic, optically reflectivestate and the semiconducting, optically transmissive state.
 2. Theoutput coupler as recited in claim 1, wherein said optical devicefurther comprises a thin film of said material sandwiched betweensubstrates which are transparent to said luminous energy.
 3. The outputcoupler as recited in claims 1 or 2, wherein said means for applying anelectric field across said optical device material comprises twosubstantially transparent juxtaposed metallic electrodes deposited onsaid material and locating said material therebetween, and acontrollable source of electric voltage connected across saidelectrodes.